1. Field of the Invention
The present invention relates to a projection optical system, and more specifically, it relates to a projection optical system suitable for a projection type image display apparatus which projects images to be displayed on an image display panel, of which a liquid crystal display element or a digital micro-mirror device is representative, onto a screen surface in a magnified scale.
2. Description of the Related Art
As a projection optical system to be used for a projection type image display apparatus (a projection) which projects images onto a predetermined surface (a screen surface), a projection optical system which can project images obliquely onto the screen in order to perform image projection without obstructing the visual field of viewers is known. Generally, since so-called trapezoidal distortion occurs when projecting images obliquely onto the screen, there is known a projection optical system which is made to correct this trapezoidal distortion (for example, U.S. Pat. Nos. 5,820,240 and 6,123,425 and Japanese Patent Application Laid-Open No. H10-282451). In Japanese Patent Application Laid-Open No. H09-304733, a projection optical system for rotating a projected image surface using coaxial rotationally symmetric f-θ lenses in a state in which there is no trapezoidal distortion of the projected image due to rotationally decentering a lens unit around a stop is disclosed. Further, in Japanese Patent Application Laid-Open No. H10-282451, a projection optical system for correcting the trapezoidal distortion at a fixed projection angle by using a decentered aspherical surface is disclosed.
Meanwhile, in non-coaxial optical systems, a design method or method of calculating paraxial amounts such as the focal length is known (for example, Japanese Patent Application Laid-Open No. H09-005650). Further, it has become clear that it is possible to construct optical systems with sufficient correction of aberrations by introducing the concept of reference axis and using asymmetric aspherical surfaces as the structural surfaces (for example, U.S. Pat. Nos. 5,825,560, 5,847,887, 6,021,004, 6,166,866, 6,292,309, 6,366,411, and 6,522,475). Such non-coaxial optical systems are referred to as an off-axial optical system. Generally, such off-axial optical systems are defined as optical systems which, when the path of the light ray passing through the image center and the pupil center is defined as the reference axis, include curved surfaces (off-axial curved surfaces) whose surface normal at the intersection of the structural surface with the reference axis is not on the reference axis. Here, the reference axis takes on a bent shape. In such off-axial optical systems, the structural surfaces are ordinarily non-coaxial, and there is no vignetting at the reflective surfaces. Thus, it is easy to construct an optical system using the reflective surfaces. Moreover, a compact optical system with a broad field angle can be constructed by forming an intermediate image within the optical system. In addition, the light path can be guided relatively freely while using a front-stop optical system, so that a compact optical system can be constructed. Utilizing these characteristics, an optical system in which the trapezoidal distortion at the fixed projection angle is corrected using curved rotationally asymmetric reflective surfaces has been proposed (for example, US Patent Application Publication No. 2002-008853 and Japanese Patent Application Laid-Open No. 2000-089227).
Further, a recent projection type image display apparatus comprises a projection optical system in which a projected image can be moved in parallel on a predetermined image surface with no trapezoidal distortion, in order to secure the visual field of viewers and improve the degree of freedom for setup positions for the projection type image display apparatus.
The methods for moving the projected image in parallel are broadly divided into two types. One method relates to a digital correction in which images to be displayed on an image display panel is distorted in advance in a trapezoidal shape so as to offset the trapezoidal distortion which is produced on a predetermined image surface (on a projection surface). In this case, in order to produce virtually an inversed trapezoidal distortion on the image display panel, the number of pixels on which images are to be actually displayed is decreased in comparison with the amount of original information, which results in deteriorating images. Further, when image information created in a personal computer or the like is displayed, there are many cases in which characters or figures or the like are displayed on the periphery of the image surface. However, the small-sized characters or figure patterns may be unrecognizable due to pixel faults of the digital correction.
Another method relates to a lens shift. In this case, the central axis of the image display panel and coaxial projection lenses is shifted relatively in parallel, such that the projected image is moved in parallel on the predetermined image surface. Conventionally, in a front projection type projector, this method is used to secure the observer's view. More specifically, in the front projection type projector, the center of the display screen is fixed above the optical axis of coaxial lenses in the projector using approximately a half of an effective field angle of the projection optical system with respect to the optical axis.
Further, in such a projector, the parallel decentering amount of the coaxial lenses on the image display panel is made to be variable, to thereby move the projected image on the predetermined image surface. However, if the amount of the movement of the projected image increases, the size of the projection optical system is magnified. Thus, the amount of the movement is naturally small, and the tilt angle of the projection to the predetermined image surface becomes small. As a result, it is difficult to secure sufficiently the observer's view and to obtain the degree of freedom for setup positions of the projection type image display apparatus.
In addition, the size of a region (an effective projection region) on which the projection can be performed is magnified or demagnified in proportional to a zoom ratio of the projection optical system even though any method is selected. Therefore, the ratio of the image size to the movement amount is constant. As a result, when the size of the image surface is smaller (a telephoto side), the degree of freedom for the setup positions of the apparatus is compromised.
In Japanese Patent Application Laid-Open No. H09-304733, the projection optical system uses a combination of coaxial rotationally symmetric f-θ optical systems, but it has a rather narrow field angle. Moreover, in conventional projection lenses, the light amount is reduced as the field angle from the optical axis up to the periphery of the image surface becomes large, so that there are greater differences in the brightness on the projected image surface when a broader field angle of the projection lens is used.
For this reason, such projection lenses are not suited for projectors which necessitate a broad field angle and a bright image surface. In Japanese Patent Application Laid-Open No. H10-282451, the trapezoidal distortion is corrected with the decentered aspherical surfaces. However, since it is a projection optical system which can correct the trapezoidal distortion only at a fixed projection angle, the shift amount of the image surface is small and it is not telecentric with respect to the liquid crystal panel. In Japanese Patent Application Laid-Open Nos. 2001-255462 and 2000-089227, the projection optical systems are disclosed in which the correction of the trapezoidal distortion over a broad field angle is accomplished using curved rotationally asymmetric reflective surfaces. However, they are projection optical systems in which the trapezoidal distortion can be corrected only at a fixed projection angle.